Many years ago, before mobile internet was reasonable and before wireless internet was available, and before even electrical outlets were something which could be counted on to be present on trains, I took a 6 hour train ride. I had no laptop. I printed out, on paper, the entire source code of the project I was working on, and brought a red pen. I read through the whole thing, from start to finish. Many subtle bug fixes, refactorings, and efficiency improvements were made that day.

This used to be a more common practice before the ubiquity of powerful IDEs and related code-reviewing utilities. It was called "desk checking".

In those days formal code reviews were also much more rare, and I for one am grateful that production code is reviewed as a matter of course.. but I do think there's something about the physical printout and kinesthetics of using a real writing implement that changes the way you read over the code. Maybe I'm a little old-fashioned that way, I also take notes with paper and pencil pretty regularly (though not exclusively).

I think everyone should do a full desk check like that at least once in their life to feel the difference, it might even improve the way one approaches tool-based code reviews (I think it does).